Praised Italian director, the man behind films classics La Strada, La Dolce Vita, 8½, Roma and Amarcord, actually had plans for an adaption of Lee Falk's Mandrake The Magician. Believe it or not!
The project was scrapped before pre-production, sadly.

I don't know much about it, why he got the idea in the first place or why he never tried it. But I know that Fellini didn't only do heavy stuff, he dealt with some funny and twisted themes as well.

How the film would have been? Not sure. But I imagine the hero to look something like Hugh Jackman in The Prestige, but with a moustache.

What about the special effects? Don't worry, it could have been done without it. Mandrake using his hypnotic powers doesn't require a full set of cgi. Filmmakers back in those days had other tricks. Just look at how Hitchcock did Vertigo and showed James Stewart's fear of heights.
Tilt the camera, zoom in/out or whatever. Those artist could make it work.

Or maybe it's a good thing that Fellini (and Hitchcock) never tried action/adventure stuff?